Cloud draft structure and trace gas transport

Abstract

Field observations obtained during the second NASA Amazon Boundary Layer Experiment (ABLE 2B), and two-dimensional moist cloud model simulations are used to determine the dominant transport pathways within a continental tropical squall line. A surface-based network triangle provided the focus for a multi-instrumental sampling of the May 6, 1987, squall line which propagated through the central Amazon basin at a rate of 40–50 km h⁻¹. Extensive use is made of the vertical distribution of specific trace gases that are representative of the prestorm and poststorm environment. One-dimensional photochemical model results suggest the observed poststorm changes in ozone concentration can be attributed to convective transports rather than photochemical production. Two-dimensional cloud model results detail the dynamic and thermodynamic attributes of the simulated squall convection. The well-mixed moist troposphere in which the observed squall system developed may have hindered strong downdraft development. Parcel trajectory analyses are conducted to investigate the flow patterns of convective transports. A significant proportion (> 50%) of the air transported to the anvil region originated at or above 6 km, not from the boundary layer via undilute cores. The presence of a midlevel inflow and a strong melting layer at 5.5 km reduced the vertical development of the core updraft and aided in the maintenance of a rotor circulation. The predicted absence of more than one active cell in the model cloud field, the lack of a well-organized downdraft in the presence of model estimated net upward mass flux, and the initial wind profile suggest the May 6 squall line was unicell in character.